The system () is typically a slow-fast system with \(x_1\)  the fast and \(x_4\)  the slow variable [1]}, [2]}.
The fast dynamics is described by
\(\dot{x_1} &=& - k_1 x_1 (k_4 - x_4 + x_1) + k_2 (x_4-x_1) \) 
